This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Appln. No. 199 59 005.2 filed in Germany on Dec. 8, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to the field of measurement of small oil concentrations in water. It proceeds from a method and a device for oil-in-water measurement according to the preamble of claims 1 and 6.
High-pressure separator tanks have recently been developed for offshore oil production which are suitable for separating the phases of sand, water, oil and gas directly at the seabed. According to statutory provisions, the water pumped back into the sea may have an oil concentration of at most a few 10 ppm. Although conventional capacitive or optical oil-in-water sensors can achieve such high measuring sensitivity in principle, it is not possible to achieve the required freedom from maintenance, long service life and pressure and temperature resistance.
Such a device for determining a low oil concentration in water is disclosed in U.S. Pat. No. 4,137,494. The measuring cell consists of an oil-absorbing diaphragm with electrodes mounted on both sides. Oil molecules contained in the water penetrate into the diaphragm, displace ions embedded there and increase the electric resistance of the diaphragm. The change in resistance is then a measure of the oil concentration in the water flowing through. In order to raise the measuring sensitivity, a water sample has to be pumped into a closed circuit repeatedly through the diaphragm in order to absorb all the oil there. The method has substantial disadvantages. As soon as the total amount of oil accumulated reaches a saturation value, the diaphragm must be disposed of or flushed with a solvent for the purpose of regeneration. Consequently, periodic maintenance intervals are stringently required, and this renders difficult or impossible installation at inaccessible sites, for example on the seabed. The measurement can also be falsified by a variable ion concentration in the water.
It is the object of the present invention to specify a method and a sensor for oil-in-water measurement in the case of which it is possible to implement a high measuring accuracy in conjunction with a greatly reduced maintenance requirement. This object is achieved according to the invention by means of the features of claims 1 and 6.
The solution according to the invention consists in a method and a device for oil-in-water measurement having a measuring cell whose impedance is a measure of an oil concentration in the water flowing through or in an accumulation filter, the measuring cell being automatically calibrated and/or flushed at recurring time intervals. During calibration, a current reference impedance value of the measuring cell is measured with the aid of a known reference liquid. The measuring cell is automatically cleaned of oil residues during flushing. Both self-calibration and self-flushing are carried out by a suitably designed or programmed electronic control system. Advantages are, inter alia, a reduction in signal drift, an improved long-term reliability and a long service life without monitoring or filter exchange. Overall, for the first time, an oil-in-water sensor is specified which is suitable for use at poorly accessible locations in oil production or the like.
In accordance with an exemplary embodiment, the measuring cell is operated in an alternating fashion in a long measuring cycle (for example 10 minutes) and a short flushing cycle (for example 1 minute) and/or an even shorter calibration cycle. The measuring cell preferably has two or more accumulation filters whose measuring cycles supplement one another such that a measuring signal can be determined in the measuring cell largely at any instant. This delivers a high degree of availability of the measuring cell for online measurements.
One exemplary embodiment relates to an oil-in-water sensor having a cross-flow filter whose output for oil-enriched water is connected to a capacitive measuring cell during the measuring cycle and whose output for cleaned water is connected to a capacitive measuring cell during the calibration cycle. The measuring sensitivity can be raised and the calibration liquid can be provided in the simplest way by the separating filter.
In another exemplary embodiment, there is arranged in a capacitive measuring cell an oil accumulation filter which can be flowed through in the reverse direction for flushing purposes by a valve-controlled line system. Owing to the reversal of direction during the flushing operation, oil residues or a filter cake on the filter outer surface, that is to say on the inlet side during the measuring cycle, can also be flushed out very efficiently with oil-contaminated water.
In an important exemplary embodiment, the oil accumulation filter is a cylindrical ring-type filter made from porous ceramic or polyethylene fibrid which has at least one radial and one axial connection. The cylindrical form creates a mechanically robust, compact and simultaneously large-area accumulation filter. In addition, it is possible by segmenting the electrodes on the outer and inner cylinder outer surface to implement a plurality of filters on a common ceramic body which run through measuring cycles which supplement one another in time, intermittent flushing cycles and/or cycles for determining offset. The self-flushing and self-calibration can thereby be implemented in a single component.
Further designs, advantages and applications of the invention follow from the dependant claims and from the description now following, in conjunction with the figures.